The emissions of greenhouse gases to the atmosphere are increasing at an alarming rate, and among them carbon dioxide emissions, as a result of the increased use of fossil fuels, are a major contributor to global warming and climate change, leading to international efforts to develop low carbon energy approaches which are less dependent on fossil fuels. However, the relative costs of alternative fuels to fossil fuels present a significant disadvantage, leading to efforts to develop technologies which during the transition to low-carbon economy can use fossil fuels but without the serious effects of greenhouse gas emissions. For industrial processes, such as power generation, these efforts include improved methods of capturing carbon dioxide from the processes and increasing the efficiency of the power generation process. In the case of power generation, these methods are pre-combustion carbon dioxide capture, post-combustion carbon dioxide capture and oxy-fuel combustion with carbon dioxide capture. However, each of these methods has associated disadvantages of capital and operational costs of the carbon dioxide capture plants, and in the case of oxygen-blown gasifiers or oxy-fuel processes, the additional high costs of producing oxygen from known cryogenic air separation units. In addition to high costs of construction and operation, other disadvantages are known in relation to each of these methods, including the complexity of the technology and associated risks, and low energy efficiency compared with previous plant operation, particularly for power generation.
Proposals have been made for integration of closed Brayton cycles with a nuclear heat source, or with a solar heat source. However, each of those sources presents inherent disadvantages based on the nature of the source, and significant cost disadvantages, so that the use of a fossil fuel for the heat source remains attractive if the carbon dioxide emissions can be substantially reduced or eliminated.
It has now been found that many of the disadvantages identified above, and others, can be addressed and overcome by a simple, efficient, compact and low-emission process in a pressurized combustion system which is constructed to interface with an energy conversion system, in particular a closed supercritical carbon dioxide Brayton cycle system, or similar systems, such as, but not limited to, systems for power generation. The system and method of the invention provide for integration of a highly efficient near-zero emission pressurized oxy-fuel combustion process with the energy conversion system. The combustion systems and method of the invention can be operated with fossil fuels in their various forms, i.e. as gaseous, liquid or solid fuels.
In the system and method of the invention, the heat is provided to the energy conversion system by an interface heat exchanger, and the temperature of the combustion, and hence of the combustion product stream passing to and through the heat exchanger, can in part be moderated and controlled by the mass flow rates of the fuel and oxygen as well as selective recirculation into the combustor of part of the flue gas stream leaving the heat exchanger, so as to meet the thermal energy requirements of the energy conversion system at the heat exchanger.
It has further been found that additional efficiencies can be achieved by use of the flue gas stream to preheat the incoming fuel supply and the incoming oxygen supply. Depending on the configuration of the energy conversion system, surplus heat from that system can be used to preheat the incoming fuel and oxygen supply for the combustion system. Also further efficiencies can be achieved by using the flue gas stream to drive a prime mover connected to the circulation pump. In addition, the combustion system's high pressure operation in excess of carbon dioxide supercritical pressure, avoids the energy need for carbon dioxide exhaust stream compression for storage or use. Also the pressurized flue gas exhaust stream can be purified using less energy intensive technologies such as membranes.